Well, it sucks that he's not being clearer, but you can work with what you've been given. So, he doesn't like the idea of this happening often. Maybe he's afraid that, with a long distance lover that you have to fly to visit, he'll be missing you for longer periods of time than with someone closer. Overnights will be a given, and many newly poly people have a hard time with that. Maybe he's afraid you'll end up living two lives, and he'll lose you often. Maybe he's thinking about the expense... will he be expected to bear a portion of the plane ticket costs? How often do you and he go on vacations... will you be investing vacation time and money in trips to see your lover that will take away from your ability to get away from it all with him?
I would sit him down, tell him that you need this to work for both of you and so you need to understand what the problem is, and then bring up these possible factors and see if any rings true for him. Ask him what a workable compromise would be.
Of course, he could just be jealous. Maybe he would've had the same reaction with a lover in your same town... being poly doesn't make you immune from natural human hypocrisy... it's not an excuse, it's something he needs to work through, but it's not uncommon.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.